High output therapeutic ultrasound transducer

ABSTRACT

A therapeutic ultrasound delivery system, comprises: a catheter body, a plurality of axially spaced-apart hollow cylindrical vibrational transducers disposed along a length of the catheter body, a first spring connector wrapped around the outer surfaces of the vibrational transducers, where the first spring connector exerting an inward pre-stress on the outer surfaces of the vibrational transducers; and a second connector disposed in contact with the inner surfaces of the vibrational transducers. The second connector may comprise a spring coil, particularly a spring coil having multiple deflection points which contact an inner surface of the cylindrical vibrational transducer.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 09/813,277, filed Mar. 20, 2001 now U.S. Pat. No. 6,508,775,which was a continuation-in-part of U.S. application Ser. No. 09/531,027(U.S. Pat. No. 6,432,068 B1), filed Mar. 20, 2000, the full disclosureswhich are incorporated herein by reference in its entirety for allpurposes.

BACKGROUND OF THE INVENTION

The present invention is related to medical devices and systems,particularly therapeutic ultrasound systems.

Percutaneously introduced catheters having ultrasound transducersthereon can be used to deliver localized doses of therapeutic ultrasoundenergy to various sites within a body. Such systems are ideally suitedfor treating or preventing pathological conditions such as arterialrestenosis due to intimal hyperplasia.

To achieve a high level of therapeutic effectiveness, a high amplitudeof ultrasound vibration is required. Unfortunately, the acoustic outputfrom a conventional transducer design is typically limited by theinherent properties of the piezoelectric material which forms thetransducer. Specifically, when operating typical piezoelectric ceramictransducers at high vibrational amplitudes, the ceramic tends tofracture. This transducer failure is caused by the high tensile stresseswithin the ceramic material during transducer operation, and the problemis exacerbated by the fact that although piezoelectric ceramic materialstend to have high compressive strengths, they have relatively lowtensile strengths.

A further problem common to existing catheter-based ultrasound systemsis that they lack the necessary flexibility to negotiate tortuous pathsthrough body lumens. This is especially true when such systems comprisea plurality of axially spaced apart ultrasound transducers disposedalong the length of the catheter body. In such cases, the catheterflexibility is unfortunately influenced both by the number and size ofthe conductors that are used to interconnect the various transducers.

A further problem common to existing catheter-based ultrasound systemswhich use a plurality of ultrasound transducers is the difficulty inindividually wiring each of these transducers, since a large number ofindividual wires leading to each of the transducers typically results ina rather bulky system.

BRIEF SUMMARY OF THE INVENTION

The present invention provides ultrasound and other vibrationaltransducer systems comprising a vibrational transducer, typically anultrasound transducer, which can be operated at very high vibrationalamplitudes without failure. As such, the present invention providessystems to prevent the ultrasound transducer, which preferably comprisesa ceramic piezoelectric material, from breaking apart at high amplitudeoperation. The present ultrasound transducer system is ideally suitedfor use in a catheter based therapeutic ultrasound energy deliverysystem.

In a preferred aspect, the present invention comprises a piezoelectricceramic ultrasound transducer having a restraint received therearound.The restraint is dimensioned or otherwise formed to have a structurewhich exerts a compressive pre-stress on the piezoelectric ceramictransducer element where the stress can be maintained during theoperation of the transducer. Advantageously, the compressive pre-stressprovided by the restraint operates to prevent tensile failure of theceramic transducer at high acoustic output.

In a preferred aspect, the strength of the compressive pre-stressprovided by the restraint on the transducer is approximately equal tothe tensile strength of the transducer element. As will be explained,when this occurs, the restrained transducer can provide approximatelytwice the acoustic output of a comparable un-restrained device beforetensile failure occurs.

In one exemplary aspect, the strength of the compressive pre-stressprovided by the restraint is approximately half-way between the tensilestrength and the compressive strength of the ceramic transducermaterial. (Stated another way, the strength of the compressivepre-stress provided by the restraint is approximately equal to theaverage of the tensile strength and the compressive strength of theceramic transducer material). As will be explained, when this occurs,the restrained transducer can be operated at a significantly increasedoutput amplitude without failure.

In various preferred aspects, the compressive pre-stress provided by therestraint is just high enough to permit operation of the device withouttensile failure at an output amplitude determined to be safe andeffective for treating or preventing a pathological condition such asarterial restenosis due to intimal hyperplasia. In these preferredaspects, the required thickness and stiffness (as described below) ofthe restraint may be preferably kept to the minimum necessary to meetthe acoustic output requirements, thereby minimizing the size of thedevice, and minimizing the requirements of the electrical drivecircuitry, while maximizing the efficiency of the device in convertingelectric power into acoustic power.

In preferred aspects, the restraint may comprise a tensioned wire orfilament(s) which is/are wrapped around the transducer. In otheraspects, the restraint may comprise a jacket having an inner diameterwhich is initially fabricated to be slightly smaller than the outerdiameter of the transducer. The jacket is then stretched to expand to alarger diameter such that it can just be received over the transducer.The transducer is then inserted within the expanded jacket, and thejacket is then allowed to contract such that it exerts a compressivepre-stress on the transducer. Systems for fabricating the jacket from ashape memory metal such as a nickel Titanium alloy (e.g.: Nitinol™) arealso set forth.

The transducer is preferably cylindrically shaped, and may have anoptional central longitudinal bore passing therethrough, with the boredefining an inner surface of the transducer. In various aspects, theinner and outer surfaces of the transducer are covered in whole or inpart by an electrode. In alternative aspects, the opposite longitudinalends of the transducer are covered in whole or in part by an electrode.In alternate embodiments of the invention, the transducer is formed froma series of alternating annular shaped polymer and piezoelectric ceramicrings, commonly referred to as a piezoelectric stack.

In a preferred aspect of the invention, the vibrational mode of thetransducer is a relatively low frequency “breathing mode”, wherein thecircumference of the cylinder oscillates around a nominal value, and thestress within the ceramic is predominantly in the tangential direction.In this case, tensile stress from the vibration of the transducer whichmay otherwise lead to failure can be balanced by compressive pre-stressin the tangential direction applied by a wrapped jacket type restraint.

In an exemplary aspect, the transducer may be made of a PZT-8, (orPZT-4) ceramic material, but other piezoelectric ceramics,electro-strictive ceramic materials, or non-ceramic materials such aspiezoelectric crystals may be used as well.

In the aspect of the invention in which a wrapped restraint is used, thetensioned member wrapped around the transducer may be a metal wire,metal or polymeric braid, mono-filament polymer, glass fiber, or abundle of polymer, glass or carbon fibers. Wires may have circular crosssections or be formed as a ribbon or square wire. In various aspects,the wire is placed under tension when initially wrapped around theultrasound transducer so as to maintain the compressive pre-stress onthe transducer. Alternatively, the tension may be introduced after thewrapping is applied using thermal, chemical, mechanical or other type ofprocess.

Suitable materials which may be used for either of the wrapped orjacket-type restraints described herein include, but are not limited to,high tensile strength elastic material selected from the groupconsisting of steel, titanium alloys, beryllium copper alloys, nickel,titanium and other shape memory allows (e.g.: Nitinol™), and epoxyimpregnated kevlar, glass, polyester or carbon fiber. In one exemplaryembodiment of the invention, the restraint comprises a 0.001″×0.003″Beryllium Copper alloy ribbon wire having a tensile strength of 150,000psi or greater, wrapped around the transducer under 0.25 lbs of tension.

In aspects of the invention where the restraint comprises a wire orribbon wire, the restraint may comprise multiple layers of wire orribbon wrappings using thinner ribbon or smaller wire than would be usedfor a single layer of wrapped restraint. An advantage of using suchsmaller diameter wire or thinner ribbon wire would be that reducedbending stress would be experienced during the wrapping process, therebypermitting the wire or ribbon to be tensioned to a higher average stresswithout breaking. This in turn would allow a higher compressivepre-stress to be applied to the ceramic transducer element using athinner and less stiff restraint than would instead be required for asingle layer wrap of the same material.

In those aspects of the invention where the restraint comprises a wire,ribbon wire, or other fiber under tension, the wire restraint may befixed in place on the surface of the transducer by gluing, soldering orwelding, with the compressive pre-stress being maintained during theoperation of the transducer. Such fixation could be continuous or onlyat spaced apart points or regions along the contact length between therestraint and the transducer.

The use of a beryllium copper alloy wire as the restraint has numerousadvantages including its high tensile strength, (typically 150 kpsi orgreater), corrosion resistance and conductive properties. A furtheradvantage is that a beryllium copper alloy wire is easily solderable. Assuch, it may be soldered both to an outer surface of the transducer, andbetween adjacent wraps around the transducer without the need for aspecial solder tab. In addition, a beryllium copper alloy wire caneasily be soldered at temperatures below the Curie temperature of theceramic transducer material, (which is about 300° C. for PZT-8 ceramic).Typically as well, a beryllium copper alloy wire has a tensilestrength/modulus of elasticity on the order of 190 kpsi/19Mpsi={fraction(1/100)}. This advantageous ration is similar to that of stainless steelwhich typically has a tensile strength / modulus of elasticity on theorder of 300 kpsi/30Mpsi={fraction (1/100)}.

In the aspects of the invention where the restraint comprises a jacket,such jacket may be made from a very high strain limit material havinggood elastic properties and high tensile strength. Such a jacket couldfirst be formed and then expanded to be slipped over the transducer andthen allowed to recover, thereby radially compressing the transducer. Ifinstead fabricated from Nitinol™, the jacket can be formed and thenexpanded to be slipped over the transducer. If maintained at asufficiently low temperature, the jacket will maintain its expanded sizeas it is placed over the transducer. When the temperature is allowed torise above a critical value the jacket material will contract, therebyapplying compressive pre-stress to the transducer.

In preferred aspects, a composite polymer is applied over the outside ofthe restraint. The composite polymer is adapted to dampen longitudinalaxis vibrations, to provide an electrical insulating layer and toprovide a convenient surface to which an outer jacket of the cathetermay be attached. Suitable materials for such a composite polymerinclude, but are not limited to, materials selected from the groupconsisting of high strength adhesives such as epoxy or cyano-acrylate,and polymers such as heat-shrinkable PVDF, polyester, nylon, Pebax, PVDFor polyethylene.

In preferred aspects, the ultrasound transducer is cylindrical in shapeand may further comprise a longitudinally extending bore therethrough.When air is disposed within this bore, the ultrasound energy emitted bythe transducer will be directed predominately radially outwards, sincevery little ultrasound energy passes from the dense ceramic transducerinto the low density air. Thus, the efficiency of the transducer can beenhanced, providing an ideal transducer system for mounting on acatheter.

In various preferred aspects, a plurality of vibrational transducers areprovided in the present catheter system. Preferably, such transducersare axially spaced apart along a length of the catheter body. In thisplural transducer system aspect of the invention, the transducerspreferably comprise hollow cylinders (i.e.: a cylinder having alongitudinally extending bore passing therethrough in an axialdirection, as described above). These transducers preferably have innerand outer surfaces which are metallic and at which an electric voltageis applied, thereby driving transducer operation.

In accordance with the present invention, the restraint which may bewrapped or otherwise disposed around these transducers may comprise acontinuous element extending over a plurality of successive transducers.Preferably, such a restraint extends over two, or more preferably three,or most preferably all of the axially spaced apart transducers in theprobe or catheter.

In preferred aspects, such a restraint may comprise a flexible memberwhich may comprise one or more wires or fibers having a spring or helixshaped or serpentine or zig-zag shaped structure.

In one preferred aspect, the restraint comprises a “spring connector”which is wrapped around (and extends over) a plurality of successivetransducers, and exerts an inward compressive force on successivetransducers.

As stated above, the preferred restraint may be wrapped around the outersurfaces of the successive axially spaced-apart transducers. Such arestraint exerts an inward pre-stress on the outer surfaces of thevibrational transducers such that transducer output can be increased,while simultaneously decreasing the likelihood of transducer failure. Itis to be understood that reference herein to an outer “spring connector”is not limited, but is instead defined to include any form of flexiblerestraint which exerts an inward pre-loading on a plurality of axiallyspaced apart transducers.

In preferred aspects, the inward pre-stress exerted by the restraintreceived over the outer surfaces of the successive transducers is about25% to 75% of the breaking (i.e. tensile) strength of the transducers.

The inward pre-stress exerted by the restraint (which may be wrapped orotherwise disposed around the outer surface of the transducers) may alsobe: (a) at least equal to the tensile strength of the transducers, (b)greater than the tensile strength of the transducers, and less than theaverage of the compressive and tensile strengths of the transducers (ie:½ way between the compressive and tensile strengths of the transducers),or (c) approximately equal to the average of the compressive and tensilestrengths of the transducers (ie: ½ way between the compressive andtensile strengths of the transducers).

It is to be understood that these ranges for the inward pre-stressexerted by the restraint wrapped or disposed around successivetransducers will be most preferred when an inner connector, and maycomprise a spring structure (which is received within the hollow boresof the successive transducers) exerts little or no appreciable outwardpre-loading on the inner surfaces of the transducers. In preferredaspects, such an inner connector may comprise one or more wires orfibers having a spring or helix shaped or serpentine or zig-zagstructure. In a most preferred aspect, the inner connector comprises aspring coil. Should the inner connector instead exert an outwardpre-loading, the range of inward pre-loading exerted by the restraintcan be increased accordingly to compensate.

While the spring coil or other inner connector will preferably exertlittle or no appreciable outward force on the inner surfaces of thetransducer, it is still necessary that the conductor establish anadequate electrical contact with the transducer in order to power andoperate the transducer. Thus, the spring coil or other inner connectorshould preferably apply a small radially outward force in order tomaintain electrical contact. As the transducers become smaller, however,typically having inner passage diameters below 0.75 mm, often below 1.0mm, the size of wire used for establishing the inner connection becomesmaller as well. Such smaller diameter wire, when wound into a coil,will be able to exert less outward radial force with the inner electrodesurface, thus risking intermittent breaks in electrical contact. Apreferred approach for enhancing electrical contact of spring coil innerconnectors is to provide multiple deflection points in the wire prior tocoiling. Such deflection points provide enhanced electrical contact withthe inner surfaces of the vibrational transducers, greatly reducing oreliminating the risk of breaks in electrical contact.

Optionally, the restraint (which is wrapped around the outer surfaces ofthe transducers) can be attached to the outer surfaces of thetransducers by a variety of techniques. These include, but are notlimited to, gluing, soldering and welding. Alternatively, (or inaddition to the foregoing attachment techniques) the restraint can beheld in a fixed relation to the outer surfaces of the transducers by itsnatural tendency to contract or “re-coil” around the transducers.Specifically, the restraint may comprise a spring (or other shaped)connector which can be unwound such that it increases in diameter to thedegree that it can be slipped over the transducers (while in itsexpanded state). Thereafter, the spring connector can be simply left tonaturally “re-coil”, such that it contracts around the outer surfaces ofthe transducers, and thereby exerts an inward pre-loading on thetransducers. In this aspect, the natural (unexpanded) diameter of thespring connector is slightly smaller than the outer diameter of thetransducers.

The use of the present restraint, which may comprise a spring connectordisposed around the outer surface of the transducers offers manyspecific advantages, including, but not limited to, the following.

First, the natural tendency of the spring to contract operates to exerta desired inward pre-loading force on the transducers, thereby offeringthe advantages of increased output with reduced likelihood of transducerfailure, as explained in reference to the various “restraints” describedherein.

Secondly, a single spring connecting several transducers is very easy toinstall when the catheter system is first assembled. This is due to thefact that the wire spring simply be rotated at one end (while being heldat its other end) to unwind it to a diameter sufficient that it can beslipped over the various transducers.

Thirdly, being a single continuous element which wraps around the outersurfaces of successive transducers, the present spring connectorprovides excellent ease and simplicity in system wiring as it canoperate as a single electrical contact wire between the outer surfacesof the various transducers.

Fourthly, being a spring which preferably wraps rather firmly around theouter surfaces of the spaced-apart transducers, the present springconnector advantageously also holds the transducers apart at preferredaxial separation distances, which remain constant over time.

In various preferred aspect of the invention, an inner connecting wireis disposed in contact with the inner surfaces of successivetransducers. In a most preferred aspect, the inner connecting wire is aspring which is positioned in contact with the inner surfaces of thetransducers. It is to be understood, however, that in accordance withthe present invention, the inner connecting wire need not be in the formof a spring. For example, a simple wire (or wires) can be used tomaintain electrical contact between the inner surfaces of successivetransducers. However, in the preferred case where the inner connectingwire does comprise a spring, such a spring offers numerous advantages,including, but not limited to, the following.

First, a spring electrically connecting the inner surfaces of successivetransducers to one another is very easy to install when the cathetersystem is first assembled. For example, such a wire spring may simply berotated at one end (while being held at another) to tighten it to adiameter sufficiently small that it can be slipped within the hollowinner bore of successive transducers. After it has been so positioned,it is only necessary to release the wire such that it springs back(i.e.: expands) into a larger diameter state, (thereby gently pushing upagainst the inner surfaces of the transducers).

Secondly, being a single continuous element, such a spring connectorprovides excellent ease and simplicity in system wiring as can beoperated as a single electrical contact wire connecting together theinner surfaces of the various transducers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cylindrical shaped ultrasoundtransducer having a wire restraint wrapped therearound.

FIG. 2 is a sectional view taken along lines 2—2 in FIG. 1.

FIG. 3 is a perspective view of a cylindrical shaped ultrasoundtransducer having a restraining jacket received thereover.

FIG. 4 is a sectional view taken along lines 4—4 in FIG. 3.

FIG. 5 is a perspective view of a transducer and restraint receivedwithin an outer coating.

FIG. 6 is an illustration of a system for wrapping a tensioned wirearound an ultrasound transducer.

FIG. 7A is a sectional view corresponding to FIG. 5, showing electrodesattached to inner and outer surfaces of the transducer, with therestraining jacket as shown in FIGS. 3 and 4.

FIG. 7B corresponds to FIG. 7A, but instead shows an electrode connectedto the outer surface of the transducer by way of a solder tab.

FIG. 7C corresponds to FIG. 5, but instead shows an electrode soldereddirectly to the restraining wire, as illustrated in FIGS. 1 and 2.

FIG. 8 illustrates a tool for expanding a jacket such that it can bereceived over the transducer.

FIG. 9 shows an alternate ultrasound transducer comprising alternatingannular piezoelectric and polymer sections.

FIG. 10 shows a stress vs. time plot for an unrestrained transducer.

FIG. 11 shows a stress vs. time plot for a restrained transducer,operating at less than optimal output.

FIG. 12 shows a stress vs. time plot for a restrained transducer,operating at optimal output.

FIG. 13 shows a plurality of the present transducers mounted to acatheter system for delivering therapeutic ultrasound to a patient.

FIG. 14 is an illustration of a two tubular shaped transducers (shown insectional view), with a coiled spring positioned in contact with theirinner surfaces.

FIG. 15 is an illustration of a two tubular transducers, with a coiledspring positioned in contact with their outer surfaces.

FIG. 16 is an illustration similar to FIG. 14, but showing more coilsper unit distance within each transducer than between successivetransducers.

FIG. 16A is a cross-sectional view of a vibrational transducer accordingto the present invention having a spring coil inner connector withmultiple deflection points to provide enhanced electrical contact.

FIG. 16B illustrates an exemplary method for forming a wire withmultiple deflection points which may be then formed into the spring coilconnector of FIG. 16A.

FIG. 17 is an illustration of an ultrasound catheter system according tothe present invention, showing the flexibility of the present system.

DETAILED DESCRIPTION OF THE INVENTION

A problem common to therapeutic ultrasound transducers is that whenoperating an ultrasound transducer such as a piezoelectric ceramictransducer at a very high output, the transducer will tend to fracture.Accordingly, the therapeutic effectiveness of catheter based ultrasounddelivery systems have been somewhat limited since the level ofvibrational amplitude of therapeutic ultrasound energy which theirtransducers are able to emit is limited, especially over prolongedperiods of operation.

Referring to FIGS. 1 and 2, the present invention provides a system forpreventing fracture of a ultrasound transducer, (such as a ceramicultrasound transducer), when the transducer is operated at a highoutput. In a first aspect, the present invention provides a system forpreventing tensile failure in a transducer 10, by way of a wire 14 whichis wrapped tightly around transducer 10. As can be seen, transducer 10is cylindrical shaped, having an optional longitudinally extendingcentral bore 11 extending therethrough.

In various preferred embodiments, transducer 10 has a preferred outerdiameter of 0.25 to 0.02 inches, a more preferred outer diameter of0.175 to 0.03 inches, and a most preferred outer diameter of 0.100 to0.03 inches.

In various preferred embodiments, transducer 10 has a preferred innerdiameter of 0.2 to 0.01 inches, a more preferred inner diameter of 0.125to 0.015 inches, and a most preferred inner diameter of 0.05 to 0.015inches.

In various preferred embodiments, transducer 10 has a preferred lengthof 1.0 to 0.01 inches, a more preferred length of 0.750 to 0.010 inches,and a most preferred length of 0.5 to 0.01 inches.

It is to be understood, however, that the preferred dimensions set forthherein are merely exemplary and that the present invention is not solimited to the dimensions set forth herein.

In preferred aspects, the present system provides a “high output” oftherapeutic ultrasound energy, being defined herein as being greaterthan that used for diagnostic imaging. In a most preferred aspect of thepresent invention, such “high output” is equal to or greater than 1.9 MI(mechanical index). In preferred aspects, the “high output” is achievedwith an MI less than that at which cavitation damage occurs.

In preferred aspects, the present “high output” therapeutic ultrasoundsystem is operated at an exemplary frequency range of equal to, orgreater than, 500 KHz, and less than, or equal to, 3 MHz.

Preferably, wire 14 is pretensioned when initially wrapped aroundtransducer 10 such that wire 14 exerts a compressive pre-stress ontransducer 10. Wire 14 may be made of any suitable material selectedfrom the group with mechanical properties exhibited by steel, titaniumalloys, beryllium copper alloys, Nitinol™. Wire 14 may alternativelycomprise a ribbon wire, or square wire, or a multi-strand wire. Wire 14may alternatively comprise a high tensile strength elastic material suchas epoxy-impregnated polyester, kevlar, glass or carbon fiber, in eithera mono-filament or multi-filament form.

In a preferred aspect, the tensile stress in wire 14 is about 100 kpsior higher. In one exemplary aspect of the invention, the wire is a0.001″×0.0003″ Beryllium-Copper (BeCu) alloy ribbon wire under 0.3 lbs.tension, and transducer 10 is made of a PZT-8 ceramic having a 0.050″outer diameter, a 0.010″ thickness wall, and a 0.315″ length. In thisexemplary aspect, the compressive pre-stress applied to the ceramic bythe wrapped ribbon restraint is approximately 10 kpsi, which iscomparable to the reported static tensile strength of PZT-8 ceramic at11 kpsi, and significantly greater than the reported dynamic tensilestrength of 5 kpsi.

Wire 14 is adapted to provide a compressive pre-stress on transducer 10,wherein the pre-stress is preferably maintained during the operation oftransducer 10 by the resilience of the restraining wire.

In a preferred aspect, the compressive pre-stress exerted by wire 14 ontransducer 10 is approximately equal to, or greater than, the tensilestrength of the transducer. As will be explained, when the compressivepre-stress exerted on transducer 10 is approximately equal to thetensile strength of transducer 10, a doubling of output amplitude oftransducer 10 is provided. In this preferred aspect of the invention,the stiffness of wire restraint 14 (or jacket 12) needed to provide thiscompressive pre-stress is only about {fraction (1/7)} the stiffness ofthe transducer 10, therefore it does not appreciably restrain the motionof transducer 10, as follows.

The relationship between the stiffness of restraint 12 or 14 and thetransducer 10 is established by considering that the modulus ofelasticity “Y” of restraint 12 or 14 multiplied by the cross-sectionalarea of restraint 12 or 14, divided by the modulus of elasticity “Y” oftransducer 10 multiplied by the cross-sectional area of transducer 10.

For example, using the BeCu ribbon at 19 Mpsi as wire 14, and PZT-8ceramic as transducer 10, the modulus of elasticity “Y” of the BeCuribbon is approximately 1.4 times the modulus of elasticity of the PZT-8ceramic at 13 Mpsi, when the cross-sectional area of the BeCu ribbon isonly about {fraction (1/10)} that of the ceramic (1 ml ribbon thicknessvs. 10 ml. transducer wall thickness). The relative stiffness of therestraint versus the transducer is then:$\frac{{stiffness}_{restraint}}{{stiffness}_{transducer}} = {\frac{Y_{restraint} \cdot A_{rest}}{Y_{transducer} \cdot A_{transducer}} = {\frac{19 \cdot 1}{13 \cdot 10} \approx \frac{1}{7}}}$

In one exemplary aspect of the invention, the compressive pre-stressexerted by wire 14 on transducer 10 is approximately half-way betweenthe compressive and tensile strengths of transducer 10, (e: at theaverage of the compressive and tensile strengths of transducer) therebyproviding the highest possible output without failure, (as will beexplained).

To ensure that wire 14 provides a compressive pre-stress on transducer10, it is also important to ensure that wire 14 does not simply unwrap,thereby losing its contact from the outer surface 13 of transducer 10.Accordingly, wire 14 is preferably glued or soldered against outersurface 13 of transducer 10. Alternatively, adjacent wraps of wire 14may be soldered, welded, or glued together with wire 14 being secured tothe outer surface 13 of transducer 10 by friction.

In one embodiment, wire 14 is welded, soldered, or glued to transducer10 or to adjacent wraps of wire 14 only at opposite transducer ends 15and 17. An advantage of welding wire 14 only at ends 15 and 17 is thatthis avoids relieving the stress in wire 14 due to heating or melting.As such, a circumferential weld near each of ends 15 and 17 may be usedto distribute the stress on the weld, with only a few turns of wire 14near ends 15 and 17 being under reduced stress, with the (unheated)center turns of wire 14 exerting the compressive pre-stress ontransducer 10. Alternatively, in another embodiment, wire 14 is weldedor adhesively attached along the entire length of transducer 10 betweenends 13 and 15.

Wire 14 may optionally be a ribbon wire, which has the advantage ofdistributing stress favorably over surface 13 of transducer 10, with theentire width of the ribbon in contact with the ceramic transducer 10,instead of just a narrow strip where a round wire would be in tangentialcontact with the cylindrical transducer surface. Furthermore, since aribbon wire provides the maximum amount of metal in a minimum profile, aribbon wire permits the maximum restraint with minimum increase in theoverall dimension of the restrained transducer. Furthermore, due to itsnarrow dimension in the radial direction, ribbon wire would experiencemuch lower bending strain during the wrapping process as compared around wire of comparable cross-sectional area per unit length. Anotheradvantage of ribbon wire is that it is resistant to stress relief duringthe welding process in which wire 14 is attached to outer surface 13,since the actual weld would only occupy a portion of the ribbon widthleaving a large remaining portion to sustain tensile stress while thewelding takes place.

In preferred aspects, wire 14 is selected from a material with anelongation at failure of greater than wire diameter/transducer radius,having the highest possible tensile strength. Alternatively, ribbon wire14 is selected from a material with elongation at failure of greaterthan wire thickness/transducer radius, having the highest possibletensile strength. In either case, the lowest possible modulus is desiredso that there is a minimum of restraint exerted on transducer 10.Examples of such materials include Beryllium Copper (BeCu) alloy 172,with various tempers having tensile strengths of 100-240 kpsi andelongation of 1-10%, or various stainless steel alloys, or high strengthTitanium alloys.

In a preferred aspect, wire 14 is wrapped over itself such that amulti-layer restraint is provided. An advantage of wrapping smallerdiameter wire is that it will exhibit a lower bending stress, ascompared to a larger diameter wire wrapped around the transducer.

In one preferred aspect, opposite ends 15 and 17 of transducer 10 may beelectroded. Alternatively, in another preferred aspect, an inner surface19 and outer surface 13 may instead be electroded.

In an alternate embodiment of the invention, the restraint used to exerta compressive pre-stress on the transducer comprises a jacket receivedover the transducer. Referring to FIGS. 3 and 4, transducer 10 is shownsurrounded by a restraint jacket 12 which is slipped thereover andexerts a compressive pre-stress, similar to that exerted by wire 14, aswas described above.

Jacket 12 may preferably be formed to maintain a compressive pre-stresson transducer 10 in a number of ways. In a first aspect, jacket 12 isinitially formed with an inner diameter slightly less than the outerdiameter of transducer 10. Thereafter, jacket 12 is stretched radiallyby mechanical or thermal means to expand its inner diameter to adimension such that it can just be slipped over transducer 10, withtransducer 10 received therein as shown in FIGS. 3 and 4. After jacket12 has been slipped over transducer 10, jacket 12 will then be releasedsuch that it naturally contracts somewhat around outer surface 13 oftransducer 10. Consequently, jacket 12 exerts, and maintains, acompressive pre-stress on transducer 10 during its operation.

Jacket 12 may preferably be fabricated from a high tensile strengthelastic material, including any of the exemplary materials set forthabove with respect to wire 14. Alternatively, jacket 12 may befabricated from a shape memory metal such as Nitinol™. In this aspect ofthe invention, a change in temperature will alter the size of jacket 12such that it constricts around transducer 10 after having been receivedthereover. For example, a Nitinol™ alloy can be chosen to be Martensiticat the temperature of liquid nitrogen, and super-elastic in thetemperature range from room temperature to body temperature and slightlyabove. The Nitinol™ alloy would be austenitic at elevated temperatures.Such a material can be fabricated as a thin wall tube with innerdiameter slightly less than that of the transducer. For example, theceramic transducer could have an outer diameter of 0.050″ with a 0.010″wall thickness and a 0.315″ length. The Nitinol™ tube could befabricated with an inner diameter of 0.048″ and a wall thickness of0.002″. When the Nitinol™ is cooled to liquid nitrogen temperature(˜200° C.) the Nitinol™ becomes Martensitic and is relatively easilyexpanded to an inner diameter of 0.052″, allowing it to be slipped overthe outside of the ceramic transducer. When the Nitinol™ warms up toroom temperature, it becomes super-elastic, and it attempts to recoverto its original fabricated dimensions. The recovery is limited by theceramic, but the super-elastic alloy applies a compressive pre-stress tothe ceramic, thereby preventing premature tensile failure of theceramic.

When using either jacket 12 or wire 14 as the restraint on transducer10, such restraint will preferably have a high tensile strength so thatonly a thin layer of the restraint material will be adequate, yet alsohave to have a low stiffness such that it would not unduly restrain theceramic transducer 10.

When using either a wire restraint (FIGS. 1 and 2) or a jacket restraint(FIGS. 3 and 4), the restraint is preferably received within an outercoating 16, as shown in FIG. 5. Outer coating 16 may preferably comprisea composite polymer, which operates to dampen longitudinal vibrationsand provide an electrical insulating layer. In an exemplary aspect,outer coating 16 comprises a high strength thin wall polymer such as0.001″ thick polyester or nylon polymer, attached to jacket 12 by a highstrength adhesive, preferably having at least 500 psi shear strength.

The present invention also sets forth systems for wrapping wire 14around transducer 10 such that wire 14 remains in tension. Referring toFIG. 6, two strands of wire 14 are shown being wrapped simultaneouslyaround transducer 10 as transducer 10 is rotated in direction R. In thissystem, a pair of equal weights W1 and W2 keep wire 14 under tension aswire 14 passes over pulleys P1 and P2. Since W1 and W2 are equal, thewires 14 will not produce any net bending stress on the transducer 10which could cause it to break during the manufacturing process.Alternatively, weight W2, pulleys P1 and P2 and one wire 14 may beeliminated to simplify the wrapping fixture. In this case, thetransducer 10 must be strong enough to resist the bending stress createdby the tensioned wrapping wire 14.

Longitudinally extending bore 11, as seen in FIGS. 1 to 5, maypreferably be air filled. Advantages of an air-filled bore include thefact that ultrasound energy can not be transmitted thereacross. Instead,all of the ultrasound energy emitted by transducer 10 willadvantageously be reflected off of inner surface 19, and directedradially outwardly, thereby increasing the therapeutic effectiveness oftransducer 10. Another advantage of air-filled bore 11 is that it can beused for passage of a guidewire therethrough.

FIG. 7A shows an embodiment of the present invention in which jacket 12is made of Nitinol™, with an electrical lead 22 passing under outercovering 16 and through a hole 9 passing through jacket 12 such that anelectrical lead 22 may be attached to electroded outer surface 13 oftransducer 10. Similarly, an electrical lead 24 is attached to the innersurface 19 of transducer 10 as shown. FIG. 7B shows electrical lead 22connected to electroded outer surface 13 by way of a solder tab 18. FIG.7C shows electrical lead 22 soldered directly to electrically conductivewire 14, which is in direct contact with electroded outer surface 13 oftransducer 10.

In a preferred aspect, wire 14 is soldered at ends 15 and 17 to preventunwrapping from transducer 10. The outer electrode connection may bemade by soldering directly to wire 14. As such, transducer 10 can bewrapped all the way from end-to-end with no unwrapped segment requiredfor lead attachment.

FIG. 8 illustrates a tool for expanding jacket 12 such that it can bereceived over transducer 10. The tool comprises a split mandrel 20 and atapered conical wedge 21. Conical wedge 21 is inserted into a borepassing through split mandrel 20 such that jacket 12 can be expanded. Ina preferred aspect, jacket 12 is made of Nitinol™, and the insertion ofwedge 21 into mandrel 20 is preferably done at a cool temperature suchthat when Nitinol™ jacket 12 returns to a warmer temperature, it willtend to retract radially inwards. In an exemplary aspect, Nitinol™jacket 12 will have a thickness of approximately 0.002″, offering animproved compromise in terms of strength and low restraint.

In preferred aspects, transducer 10 will be operated at a lowtemperature rise. Such low temperature rise can be achieved bymaintaining a low duty cycle, or alternatively by providing a coolingflow such as a saline infusion over transducer 10 during its operation.Preferably, a temperature rise of less than 5° C. will be achieved.Preferably, the fluid could be introduced through an annular spacebetween transducer 10 and a polyimide guidewire sleeve. Temperaturemonitoring by a catheter mounted thermistor or thermocouple can also beused.

Referring to FIG. 9, an alternate transducer system is provided withtransducer 30 comprising alternating annular sections of PZT ceramic 32and polymer 34. Transducer 30 is ideally suited to avoiding longitudinalfailure. In accordance with the present invention, transducer 30 may besubstituted for transducer 10 in any of the above described embodimentsof the present invention. For example, transducer 30 is preferablyrestrained by a wire 14 wrapped therearound, or a jacket 12 slippedthereover, the restraint used in turn being received within outercovering 16, as described.

As stated above, the strength of the compressive pre-stress provided bywire 14 or jacket 12 on transducer 10 is at least approximately equal tothe tensile strength of the transducer material and more preferably,approximately equal to the average of the tensile and compressivestrengths of the material. (ie: at a value ½ way between the tensile andcompressive strengths of the material). This is explained as follows.

Referring to FIG. 10, a stress vs. time plot for an unrestrainedtransducer is shown. Acoustic vibrations in the transducer arecharacterized by oscillation in the stress. In a conventionaltransducer, without a pre-stress, the stress oscillates around zero,alternating between compressive (positive) stress and tensile (negative)stress.

Since piezo-electric ceramic materials typically have much highercompressive strengths compared to their tensile strengths, compressivepre-stress permits higher acoustic amplitude without subjecting theceramic to tensile stress beyond its limit. Specifically, the tensilestrength of the transducer material is shown by line 50 and thecompressive strength of the transducer material is shown by line 52. (Ascan be seen, line 50 is closer to zero than line 52, thus indicatingthat the transducer is more likely to fail in tension than incompression). If the stress during one of the cycles of oscillationexceeds the tensile strength of the ceramic, then the transducer willfracture. Accordingly, when operating an unrestrained transducer, themaximum tensile stresses will equal the maximum compressive stresses.Accordingly, the maximum peak-to-peak amplitude of the oscillations inthe stress (i.e.: the difference between lines 50 and 70) will be doublethe tensile strength (i.e.: the difference between zero and line 50) ofthe transducer material.

FIG. 11 shows a stress vs. time plot for a transducer with a restraintwrapped therearound. In this aspect of the invention, the compressivepre-stress (labeled as distance “B”), (ie: the difference between zeroand line 54) is equal to the tensile strength (labeled as distance “A”),(i.e.: the difference between zero and line 50) of the transducermaterial. Thus, line 54 is at the same level as line 70. As can be seen,the application of such a compressive pre-stress to the transducerresults in a doubling of the maximum peak-to-peak amplitude ofoscillation in the stress relative to that of a comparable unrestrainedtransducer, (i.e.: the difference between line 56 and zero is twice thedifference between line 54 and zero).

FIG. 12 shows a stress vs. time plot for a transducer with a restraintwrapped therearound, operating at optimal output. In this aspect of theinvention, the compressive pre-stress applied by the restraint (line 58)is set to be positioned at an average (ie: ½ way between) the tensilestrength (line 50) and the compressive strength (line 52) of thetransducer material. As can be seen, the application of such acompressive pre-stress on the transducer effectively maximizes thepeak-to-peak amplitude of the oscillation in the stress to a levelcorresponding to the difference between compressive strength (line 52)and the tensile strength (line 50).

Accordingly, in preferred aspects of the invention, the compressivepre-stress applied to the transducer by the restraint is at least equalto, and preferably greater than, the tensile strength of the transducer.More preferably, the compressive pre-stress applied to the transducer bythe restraint is of an amplitude greater than the tensile strength ofthe material and not exceeding an average value (ie: a value ½ waybetween) the tensile and compressive strengths of the material. In anoptimal aspect of the invention, the compressive pre-stress is equal tothe average of the tensile and compressive strengths of the material.

In another preferred aspect of the invention, the compressive pre-stressapplied to the transducer is sufficient to permit reliable operation atthe desired acoustic output amplitude, without permitting tensilefailure of the ceramic and without requiring an unnecessarily stiff orbulky restraint.

As such, FIGS. 11 and 12 provide illustrations of how compressivepre-stress permits higher amplitude acoustic vibrations without stressexceeding the tensile strength limit of the ceramic compressive strengthof ceramic.

Lastly, FIG. 13 is an illustration of a plurality of the presentcylindrically shaped high output ultrasound transducers 10, with wrappedwire restraint 14 thereover, as previously described herein, mountedalong a flexible catheter 60 with spacers 62 disposed therebetween.Spacers 62 may be formed from a flexible polymer material so as topermit catheter 60 to flex between the rigid transducer (10) segments.Outer covering 16 may preferably be formed from a flexible polymer whichbonds to jacket 12, and provides a smooth outer surface for catheter 16.A plurality of optional bushings 64 are disposed between transducers 10and spacers 62, forming an air gap 65 adjacent the inner surface 66defining lumen 67 through which guide wire 68 passes, as shown. In apreferred aspect, the guidewire lumen 67 is lubricious and flexible andcontains guidewire 68 and has a fluid (such as saline) passingtherethrough to provide cooling for transducers 10. Air gap 65 operatesto direct the ultrasound energy emitted by transducers 10 radiallyoutwardly, by inhibiting radially inward ultrasound emissions. Apreferred material for guidewire lumen 67 is high density polyethylene.

FIGS. 14 to 17 show an aspect of the invention in which a plurality ofaxially spaced-apart transducers are used, with coiled springs wrappedaround their inner and outer surfaces. As explained herein withreference to other embodiments, the present invention can be used toprovide therapeutic ultrasound delivery to a patient. It is to beunderstood that although the structure of the outer restraint asillustrated herein is that of a “spring connector”, the presentinvention is not so limited. Rather, other shapes of connectors can beused, including serpentine, zig-zag and various helical structures, allkeeping within the scope of the present invention.

Referring first to FIG. 17, a catheter 100 having a plurality of hollowcylindrical ultrasound transducers 110 which are spaced apart in anaxial direction along the length of the catheter body is shown. Close-upviews of successive transducers 110 are shown in FIGS. 14 to 16 (withthe catheter body removed for ease of illustration).

Referring next to FIG. 15, a first spring connector 120 is wrappedaround the outer surfaces of successive vibrational transducers 110. Inaccordance with the present invention, first spring connector 120 exertsan inward pre-loading on each of transducers 110. In preferred aspects,the strength of this inward pre-loading is about 25% to 75% of thebreaking (ie: tensile) strength of the transducers. In alternatepreferred aspects, the strength of this inward pre-loading is: (a) atleast equal to the tensile strength of the transducers; (b) greater thanthe tensile strength of the transducers, and less than ½ way between thecompressive and tensile strengths of the transducers; or (c)approximately ½ way between the compressive and tensile strengths of thetransducers. In this aspect of the invention, first spring connector 120is a form of “restraint” (as described herein), functioning similar towire 14 or jacket 12.

Referring next to FIG. 14, a second connector 140 is disposed in contactwith the inner surfaces of successive vibrational transducers 110.Preferably, second connector 140 comprises a spring (as illustrated),but it need not comprise a spring. For example, it may comprise a simpleelectrical lead similar to lead 24 in FIG. 7A.

In various aspects, each of first spring connector 120 and secondconnector 140 may be connected to respective outer and inner surfaces oftransducers 110 by techniques including gluing, soldering, welding orbonding. Additionally, the natural tendency of a spring to “re-coil” or“spring back” into position after it has been deformed may be used toconnect the first spring connector 120 and second connector 140 to thetransducer surfaces, as follows.

First, the outer spring connector 120 can be unwound such that itexpands in diameter, and then be slipped over the transducers, andallowed to contract, tightening around the transducers. Specifically,the wrapping of first spring connector 120 around the outer surfaces oftransducers 110 may be accomplished by unwinding first spring connector102 such that expands in diameter; slipping first spring connector 120over the outer surfaces of transducers 110 when first spring connector120 is in an expanded state; and then allowing first spring connector120 to contract around the outer surfaces of transducers 110, such thatfirst spring connector exerts an inward pre-stress on the outer surfacesof the vibrational transducers.

Secondly, second connector 140 can be tightly wound such that it shrinksin diameter, and then be slipped through the bores through therespective transducers, and then be allowed to expand, tighteningagainst the inner surface of the bore through the respectivetransducers. Specifically, the positioning of second spring connector140 in contact with the inner surfaces of vibrational transducers 110may be accomplished by winding second spring connector 140 such thatcontracts in diameter; slipping second spring connector 140 through theinner surfaces of successive transducers 110 when second springconnector 140 is in a contracted state; and then allowing second springconnector 140 to expand such that it contacts the inner surfaces oftransducers 110.

In various preferred aspects, each of the first spring connector 120 andsecond connector 140 may comprise singular or multifiliar wraps. Suchmultifiliar wraps offer the advantages of increased electrical currentcarrying capacity with increasing overall stiffness.

In various preferred aspects, each of the first spring connector 120 andsecond connector 140 may be spring having a varying pitch (along itslength). Such a varying pitch spring is illustrated in FIG. 16 whichshows a tight or narrow pitch for second connector 140 in region 140 a(i.e.: within transducer 110) and a lose or wide pitch for secondconnector 140 in region 140 b (i.e.: between two transducers 110). (Itis to be understood that spring connector 120 may also have a similarvarying pitch in which its coils are spaced closer together when passingover the surface of transducer 110 and are spaced farther apart inregions between successive transducers).

Advantages of varying the spring coil pitch (for either of springconnector 120 or second connector 140) include, but are not limited to,the following. First, a greater percentage of the spring coil can bepositioned in direct contact with the outer or inner surface of thetransducer. This increases the effectiveness of the electrical contactmade by the spring coil to the transducer. Secondly, the greater pitchbetween transducers results in less electrical resistance and thereforeless energy loss due to heating.

The inner coil 140 may be modified to have multiple deflection points toprovide enhanced electrical contact with an inner surface of thevibrational transducer, as shown in FIG. 16A. There, the inner coil 140ρis bent, kinked, or otherwise deformed to have a plurality of deflectionpoints 141 along its length. Typically, the deflection points will beequally spaced apart, but such equal spacing is not necessary. Thedeflection points will be oriented so that when the connector 140 iscoiled to form a desired spring coil, the deflection points will beoriented radially outward. In this way, as the spring coil is releasedwithin the inner passage of the transducer 110, the coil will expand sothat the deflection points 141 firmly contact the inner surface 143 ofthe transducer.

Referring now to FIG. 16B, an exemplary technique for forming an innerconductor having multiple deflection points is illustrated. A suitablewire, for example, a beryllium cooper wire ribbon (0.001 inch by 0.003inch) is pulled over a shaped mandrel 200. As illustrated, the mandrelhas a hexagonal cross-section, and that other polygonal and irregularshaped mandrels could be used. The wire 202 is pulled in the directionof arrow 204 with a tensile force in the range from 20 grams to 100grams. As the wire 202 is pulled, the mandrel 200 is turned in thedirection of arrow 206 at a rate which matches the axial advancement ofthe wire 202. In this way, the deflection points 141 are formed in thewire which may then be coiled and placed into the transducer 110, asshown in FIG. 16A.

The present catheter 100 can be used for delivering vibrational energyto a patient. This can be accomplished as follows. Catheter 100 can beintroduced into a patient, with first spring connector 120 disposed incontact with outer surfaces of vibrational transducers 110, (exerting aninward pre-loading on the vibrational transducers 110), and with secondconnector 140 disposed in contact with inner surfaces of transducers110. Thereafter, transducers 110 can be energized to deliver vibrationalenergy to the patient. In optional preferred aspects, transducers 110are operated at a Mechanical Index (MI) of at least 1.9; and at afrequency of at least 500 KHz, but not exceeding 3 MHz. In optionalpreferred aspects, the inner bores of transducers 110 may be cooled witha cooling flow, which may optionally comprise saline.

1. A therapeutic ultrasound delivery system, comprising: a catheterbody; a plurality of axially spaced-apart hollow cylindrical vibrationaltransducers disposed along a length of the catheter body saidtransducers having outer and inner surfaces; an outer restraint disposedaround the outer surfaces of the vibrational transducers, the outerrestraint exerting an inward pre-stress on the outer surfaces of thevibrational transducers; and an inner spring coil connector disposed incontact with the inner surfaces of the vibrational transducers.
 2. Thesystem of claim 1, wherein the inner spring coil has multiple deflectionpoints which provide enhanced electrical contact with the inner surfacesof the vibrational transducers.
 3. The system of claim 2, wherein theouter restraint is attached to the outer surface of the vibrationaltransducers by one of the group consisting of gluing, soldering, weldingor bonding.
 4. The system of claim 2, wherein the inner connector isattached to the inner surface of the vibrational transducers by one ofthe group consisting of gluing, soldering, welding or bonding.
 5. Thesystem of claim 2, wherein the inner and outer surfaces of thevibrational transducers are metallic.
 6. The system of claim 2, whereinat least one of the outer restraint and inner connector comprises asingle filament wrap.
 7. The system of claim 2, wherein at least one ofthe outer restraint and inner connector comprises a multifiliment wrap.8. The system of claim 2, wherein the outer restraint exerts an inwardpre-stress equal to about 25% to 75% of the breaking strength of thetransducers.
 9. The system of claim 2, wherein the outer restraintexerts an inward pre-stress at least equal to the tensile strength ofthe transducers.
 10. The system of claim 2, wherein the outer restraintexerts an inward pre-stress that is greater than the tensile strength ofthe transducers, and less than the average of the compressive andtensile strengths of the transducers.
 11. The system of claim 2, whereinthe outer restraint exerts an inward pre-stress that is approximatelyequal to the average of the compressive and tensile strengths of thetransducers.
 12. The system of claim 2, wherein the vibrationaltransducers are made from the group consisting of a piezoelectricceramic, an electrostrictive ceramic and a piezoelectric crystal. 13.The system of claim 12, wherein the vibrational transducers are madefrom the group consisting of PZT-8 and PZT-4 ceramic material.
 14. Thesystem of claim 2, wherein the outer restraint is a spring.
 15. Thesystem of claim 2, wherein the inner spring coil connector pushes gentlyoutwardly against the inner surfaces of the vibrational transducers. 16.The system of claim 14, wherein the pitch of the spring is less inregions adjacent each of the individual transducers than in regionsbetween each of the individual transducers.
 17. The system of claim 2,wherein the pitch of the inner spring coil is less in regions within theinner bores of each of the individual transducers than in regionsbetween each of the individual transducers.
 18. The system of claim 2,wherein the outer restraint is made from a material having a Young'sModulus in the range of 10,000,000 psi to 70,000,000.
 19. The system ofclaim 2, wherein the outer restraint is made from a material having atensile strength in the range of 50,000 psi to 400,000.
 20. The systemof claim 2, wherein the outer restraint is made from a material havingelectrical conduction properties in the range of 9 to 100 ohms permil:ft.
 21. The system of claim 2, wherein at least one of the outerrestraint or inner connector is made from a material selected from thegroup consisting of beryllium copper alloys, steel alloys, aluminumalloys, nickel alloys, nickel titanium alloys and tungsten alloys. 22.The system of claim 2, wherein the outer restraint is disposed aroundthe outer surfaces of two successive vibrational transducers.
 23. Thesystem of claim 2, wherein the outer restraint is disposed around theouter surfaces of three successive vibrational transducers.
 24. Thesystem of claim 2, wherein the outer restraint is disposed around theouter surfaces of all of the transducers in the catheter body.
 25. Amethod of assembling a system for delivering vibrational energy to apatient, comprising: providing a catheter body; providing a plurality ofaxially spaced apart hollow cylindrical vibrational transducers disposedalong a length of the catheter body; connecting together the pluralityof hollow cylindrical vibrational transducers within the catheter bodyby: wrapping a first spring connector around the outer surfaces of thevibrational transducers wherein the first spring connector exerts aninward pre-stress on the outer surfaces of the vibrational transducers;and positioning a second spring coil connector in contact with the innersurfaces of the vibrational transducers, wherein the second spring coilconnector has multiple deflection points which contact the innersurfaces.